1. Technical Field
The present application relates generally to electronic devices having a laser scan unit (LSU), and particularly to an improved electrophotographic devices having reduced jitter and scan line variability for off-axis based LSU systems.
2. Description of the Related Art
Laser scanning systems, such as those utilized in color electrophotographic imaging devices, typically include a plurality of laser sources, each of which generates a laser beam containing data to be imaged onto a media sheet, a rotating polygonal mirror from which the laser beams reflect such that each beam impinges on a distinct photoconductive drum in a scan pattern formed by a plurality of parallel scan lines. There are some characteristics associated with laser scanning systems which adversely impact the quality of an imaged sheet.
For example, a number of motor and mirror characteristics are seen to be a root cause of scan line shift (jitter) and scan line length variation in a laser scanning unit. When jitter and scan line length variation occur, halftoned images can experience a degraded quality especially when using higher lines per inch (LPI) halftone screens. Essentially, the once per motor revolution variance interacts with the screen pattern.
One such characteristic is pyramidal angle error (PAE), which is the error associated with the incident beam angle θ from normal when viewing the polygon mirror 102 from the side. See FIG. 1. In horizontal synchronization (hsync) based systems that have two laser sources 104 on one side of the LSU, PAE causes the beams to hit the first lens of the LSU at varying locations and depending on the scan power in that lens can induce scan jitter.
Another characteristic, motor runout, is a measure of the variation in distance from the laser to the center of each facet and is caused by misaligned mounting by a distance X of the mirror center onto the motor shaft driving the mirror. In an off-axis system, this causes the line length of a scan to vary from facet to facet. Motor runout is also illustrated in FIG. 1.
Smaller sized laser printers are forcing its laser scanning units into a smaller space which leads designers to off-axis system scanning architectures. Such off-axis architectures result in the design being more susceptible to the characteristics mentioned above. Purchasing minors that have low PAE and motors that have low runout can add about $1 to $2 per laser scanning unit.
What is needed, then, is an improved LSU system which reduces or substantially eliminates scan jitter and scan line length variation.